From the past, a variety of flame retardant additives have been used in resins and elastomers for imparting flame retardance thereto. Halogen-containing compounds have been the mainstream of the flame retardant technology. Flame retardant resin compositions are generally prepared by compounding thermoplastic resins with halogen-containing compounds alone or in combination with antimony compounds such as antimony oxide. It is recently regarded problematic that these flame retardant resin compositions give off halide gases during combustion or molding. Metal hydroxides are also used as the flame retardant. In order to provide certain flame retardance, metal hydroxides must be heavily loaded at the expense of processability and mechanical strength.
Phosphorus compounds are expected as a promising candidate and have been widely used in practice. Among numerous phosphorus compounds, ammonium polyphosphate is expected to impart a high level of flame retardance when added to various resin compositions because it has a high phosphorus content and contains within its molecule a nitrogen atom which is believed to act in synergy with phosphorus.
However, ammonium polyphosphate is less water resistant in itself, and in the case of resin compositions, also gives rise to the problems of bleeding and substantially degraded electrical properties when held under hot humid conditions. For fiber treatment, ammonium polyphosphate is generally coated in emulsion liquid form. Since ammonium polyphosphate tends to agglomerate due to moisture absorption, the treatment becomes uneven so that flame retardance may develop with difficulty. A number of studies have been made to overcome these problems.
One solution is to treat surfaces of ammonium polyphosphate particles with melamine compounds such as by coating (see JP-B 53-15478, JP-B 52-39930, JP-A 61-103962 and JP-A 8-183876). These methods, however, have problems including difficult preparation, agglomeration of particles, still insufficient water resistance, and generation of formaldehyde. Additionally, since melamine compounds are less dispersible in various resins, it is difficult to disperse melamine compound-coated ammonium polyphosphate in resins.
Means for improving the water resistance and dispersibility of ammonium polyphosphate, proposed so far, include treatments with silane-derived coupling agents. See JP-B 6-6655, JP-B 6-4735 and JP-B 6-18944. These treatments impart only some water repellent effects and are insufficient to solve the problems including a lessening of electrical properties.
JP-A 8-134455 discloses to modify ammonium polyphosphate with silica fine powder surface coated with silicone oil. This treatment imparts only some water repellent effects and is insufficient to solve the problems including a lessening of electrical properties.
It was also proposed to add silicone oil and/or silicone resin and ammonium polyphosphate separately to thermoplastic resins, as disclosed in U.S. Pat. No. 4,871,795 (Pawar) and JP-A 5-39394. These methods still leave the problem that ammonium polyphosphate picks up moisture and bleeds to the surface, detracting from physical properties of resin.